1. Field of the Invention
The present invention relates to a technique for scanning originals with an image sensor, such as CCDs (charge-coupled devices).
2. Description of the Related Art
An image scanning apparatus generally illuminates an original with light and introduces light reflected from the original (or transmitted through the original) into a first-dimensional CCDs, in order to read image signals of the original from the first-dimensional CCDs. The respective elements of the first-dimensional CCDs do not have identical sensitivity characteristics. Even when light enters all the elements homogeneously, there is a scatter in the output level of the elements. Shading correction is a typical procedure to correct the scatter in the sensitivity characteristics of the respective elements of the one-dimensional CCDs. The process of shading correction scans a white reference plate (or a black reference plate) prior to the scan of the original, and generates coefficient data used for correcting the sensitivities of the respective elements of the one-dimensional CCDs, based on the output signals with respect to the reference plate from the one-dimensional CCDs. The process of shading correction corrects the output signals of an image obtained by scanning the original with the coefficient data.
It is here assumed that a plurality of originals are set on an original tray. The plurality of originals have different scanning resolutions and different widths. It is accordingly required to set a scanning angle of the one-dimensional CCDs for each original. The `scanning angle` here represents an angle of a possible shooting range of the first-dimensional CCDs. The coefficient data used for correcting the sensitivities depend upon the scanning angle of the one-dimensional CCDs. The prior art technique accordingly carries out the shading correction for each of the plurality of originals set on the original tray, prior to the scan of the original.
As mentioned above, the prior technique requires the shading correction every time when the target of the scan is moved to a next original among the plurality of originals set on the original tray. This means that the process of scanning the white reference plate and updating the coefficient data used for correcting the sensitivities should be carried out for each original. This undesirably lengthens the total time required for scanning images.